In the United States, a law for enforcing a tire air pressure alert device mounted on an automobile has been put into effect. As a tire pressure monitoring system (TPMS), a system employing RFID (Radio Frequency Identification) technique has been proposed. In the TPMS using RFID technique, a transponder (a tire pressure detection device) having a pressure detection function and mounted on a tire communicates by radio with a reader (a control unit) mounted on a vehicle body, while utilizing the RFID technique.
The value or the like measured by the pressure sensor is digitally encoded by a transponder, and the resultant digital code is sent to a vehicle side reader, using the RFID technique. Having received the digital code, the reader obtains data, such as a measured air pressure value, from the received digital code.
The pressure sensor comprises a displacing portion, such as a diaphragm or the like, subjected to mechanical displacement, and a structure for converting the displacement into a change of electrical resistance and/or electrostatic capacitance, to thereby convert the displacement due to pressure into an electric signal for determination.